1. Field of the Invention
The invention relates to an apparatus for capacitive filling-level measuring, particularly in a motor vehicle tank, comprising a primary element with measuring electrodes provided on an oblong body and forming a measuring condenser, which electrodes are surrounded at a variable level by the medium to be measured, and further comprising compensation electrodes for the formation of a compensation condenser by which the effect of various dielectric constants of the medium upon the measuring result can be compensated in an evaluation circuit to which the measuring condenser and the compensation condenser are connected.
2. Prior Art
In a known apparatus of this type for capacitive filling-level measuring, a primary element has at least two concentric electrodes, fixed on a support. In detail, one of the electrodes consists of a conductive layer applied to an oblong insulating material plate as support, while the other electrode is shaped as an oblong metal support whereon the insulating material plate with the conductive layer facing the support is fixed. An evaluation circuit in hybrid construction, which contains an oscillator, is connected to the electrodes in order to generate an electric current suitable to indicate the filling-level. The evaluation circuit is fixed on the portion of the primary element that protrudes from the medium to be measured. Since in this apparatus, which contains only a measuring condenser formed by the electrodes, for measuring the filling-level, the dielectric constant of the medium to be measured enters the measuring result, wrong indications of the filling-level may occur. This applies especially in the filling-level measuring of fuels wherein the dielectric constant may vary with varying methanol content. In order to eliminate the indication differences based thereon, it is known in connection with the initially mentioned apparatus to provide, in addition to the two measuring electrodes forming the measuring condenser, at least one reference electrode, which is also called a compensation electrode. This reference electrode is produced by a further conductive layer on the insulating material plate. It forms, together with the portion of a metal supporter sheet metal strip facing it, a reference capacity or a compensation condenser. By means of this conpensation condenser the effects of differing dielectric constants upon the measuring result can be compensated. For this purpose, the evaluation circuit comprises an oscillator whose output is connected via a resistor in each case to the measuring condenser and the compensation condenser. The two condensers are connected to two inputs of a sum-and-difference amplifier in the output of which, e.g., a quotient meter is provided.
For the aforementioned compensation of the effects of differing dielectric constants of the medium to be measured, it is assumed that the compensation condenser is always equally surrounded, i.e. as a rule completely, by the medium. To this end, the compensation electrodes of the compensation condenser have hitherto been arranged above the bottom of the container wherein the medium to be measured is contained. This arrangement, however, means that the container must at any rate not be emptied below the level of these compensation electrodes. When the compensation electrodes are arranged too low on the container, there exists, moreover, the possibility that the medium which at this place surrounds the compensation electrodes is of another composition than the medium in the area of which the measuring electrodes are arranged. This applies especially to motor vehicle tanks wherein water contained in the fuel may be deposited in the course of time. Furthermore, this arrangement of the compensation electrodes near the bottom may have the result that the capacity of these compensation electrodes is relatively small if not too bulky an arrangement is to be put up with. Since, besides, the supply lines to these compensation electrodes are relatively long, the capacity formed by the supply lines must possibly also be compensated.
The aforementioned disadvantages prevail particularly when the compensation condenser is constructed in the manner known, per se, as a measuring condenser, in which construction the two electrodes are applied to an insulating material support in the form of two stripes engaging each other in a comblike manner.